Packaging Mechanisms for Dies with Different Sizes of Connectors

ABSTRACT

Embodiments of mechanisms for testing a die package with multiple packaged dies on a package substrate use an interconnect substrate to provide electrical connections between dies and the package substrate and to provide probing structures (or pads). Testing structures, including daisy-chain structures, with metal lines to connect bonding structures connected to signals, power source, and/or grounding structures are connected to probing structures on the interconnect substrate. The testing structures enable determining the quality of bonding and/or functionalities of packaged dies bonded. After electrical testing is completed, the metal lines connecting the probing structures and the bonding structures are severed to allow proper function of devices in the die package. The mechanisms for forming test structures with probing pads on interconnect substrate and severing connecting metal lines after testing could reduce manufacturing cost.

PRIORITY CLAIMS AND CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser. No. 15/700,830, filed Sep. 11, 2017, entitled “Packaging Mechanisms for Dies with Different Sizes of Connectors”, which is a continuation application of U.S. application Ser. No. 14/728,608, filed Jun. 2, 2015, now U.S. Pat. No. 9,761,503, issued Sep. 12, 2017, entitled “Packaging Mechanisms for Dies with Different Sizes of Connectors,” which is a divisional application of U.S. application Ser. No. 13/924,215, filed Jun. 21, 2013, now U.S. Pat. No. 9,070,644, issued Jun. 30, 2015, entitled “Packaging Mechanisms for Dies with Different Sizes of Connectors”, which claims the priority of U.S. Provisional Application Ser. No. 61/798,136, entitled “Method and Apparatus for a Package Structure,” and filed on Mar. 15, 2013, and U.S. Provisional Application Ser. No. 61/791,944, entitled “Packaging Interconnect Structure Apparatus and Method,” and also filed on Mar. 15, 2013. All of the above-mentioned applications are incorporated herein in their entirety.

BACKGROUND

Semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, and other electronic equipment, as examples. Semiconductor devices are typically fabricated by sequentially depositing insulating or dielectric layers, conductive layers, and semiconductive layers of materials over a semiconductor substrate, and patterning the various material layers using lithography to form circuit components and elements thereon.

The semiconductor industry continues to improve the integration density of various electronic components (e.g., transistors, diodes, resistors, capacitors, etc.) by continual reductions in minimum feature size, which allow more components to be integrated into a given area. These smaller electronic components also require smaller packages that utilize less area and/or lower height than packages of the past, in some applications.

Thus, new packaging technologies have begun to be developed. By adopting the new packaging technologies, the integration levels of the packages may be increased. These relatively new types of packaging technologies for semiconductors face manufacturing challenges.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view of a die package, in accordance with some embodiments.

FIG. 2A shows a cross-sectional view of a die package, in accordance with some embodiments.

FIG. 2B shows a top view of die package of FIG. 2A, in accordance with some embodiments.

FIGS. 3A-3E illustrate cross-sectional views of a sequential flow of forming an interconnect substrate, in accordance with some embodiments.

FIGS. 4A-4E illustrate cross-sectional views of a sequential flow of forming packaged die, in accordance with some embodiments.

FIGS. 5A-5D illustrate cross-sectional views of a sequential flow of forming die package, in accordance with some embodiments.

FIG. 6 shows a cross-sectional view of a die package, in accordance with some embodiments.

FIGS. 7A-7E illustrate cross-sectional views of a sequential flow of forming die package, in accordance with some embodiments.

FIG. 8 shows a top view of a die package, in accordance with some embodiments.

FIG. 9A shows a top view of FIG. 5C, in accordance with some embodiments.

FIG. 9B shows a top view of FIG. 5C, in accordance with some other embodiments.

FIG. 10A shows electrical connections between a number of probing pads and bonding structures of packaged die(s), in accordance with some embodiments.

FIG. 10B shows severed electrical connections between a number of probing pads and bonding structures of packaged die(s) after electrical testing, in accordance with some embodiments.

FIGS. 11A-11C show cross-sectional views of a sequential flow of testing and bonding packaged dies during formation a die package, in accordance with some embodiments.

Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION

The making and using of the embodiments of the present disclosure are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are illustrative and do not limit the scope of the disclosure.

FIG. 1 is a perspective view of a die package 100 including a packaged die 110 bonded to an interconnect substrate 120, which is further bonded to a substrate (or package substrate) 130 in accordance with some embodiments. Two or more packaged dies can be bonded to the interconnect substrate 120. The two or more packaged dies could be different from one another. However, two or more of the packaged dies bonded to the interconnect substrate 120 could be identical. For example, two identical packaged memory dies and a packaged processing-unit die, such as central processing unit (CPU) or graphic processing unit (GPU), could be bonded to interconnect substrate 120.

Each packaged die, such as packaged die 110 includes at least a semiconductor die (not shown). The semiconductor die includes a semiconductor substrate as employed in a semiconductor integrated circuit fabrication, and integrated circuits may be formed therein and/or thereupon. The semiconductor substrate refers to any construction comprising semiconductor materials, including, but not limited to, bulk silicon, a semiconductor wafer, a silicon-on-insulator (SOI) substrate, or a silicon germanium substrate. Other semiconductor materials including group III, group IV, and group V elements may also be used. The semiconductor substrate may further comprise a plurality of isolation features (not shown), such as shallow trench isolation (STI) features or local oxidation of silicon (LOCOS) features. The isolation features may define and isolate the various microelectronic elements. Examples of the various microelectronic elements that may be formed in the semiconductor substrate include transistors (e.g., metal oxide semiconductor field effect transistors (MOSFET), complementary metal oxide semiconductor (CMOS) transistors, bipolar junction transistors (BJT), high voltage transistors, high frequency transistors, p-channel and/or n-channel field effect transistors (PFETs/NFETs), etc.); resistors; diodes; capacitors; inductors; fuses; and other suitable elements. Various processes are performed to form the various microelectronic elements including deposition, etching, implantation, photolithography, annealing, and/or other suitable processes. The microelectronic elements are interconnected to form the integrated circuit device, such as a logic device, memory device (e.g., SRAM), RF device, input/output (I/O) device, system-on-chip (SoC) device, combinations thereof, and other suitable types of devices.

Interconnect substrate 120 may be made of a semiconductor wafer, or a portion of wafer. In some embodiments, interconnect substrate 120 includes silicon, gallium arsenide, silicon on insulator (“SOI”) or other similar materials. Interconnect substrate 120 includes interconnect structures or redistribution layer(s) (RDL) to electrically connect packaged die 110 and substrate 130. RDLs are interconnect structures near the surface of die packages or on packaging structures to facilitate electrical connections. In some embodiments, interconnect substrate 120 also includes passive devices such as resistors, capacitors, inductors and the like, or active devices such as transistors. In some embodiments, substrate 130 includes additional integrated circuits. Interconnect substrate 120 may further include through substrate vias (TSVs) and may be an interposer. In addition, the interconnect substrate 120 may be made of other materials. In some embodiments, interconnect substrate 120 also includes bismaleimide triazine (BT) resin, FR-4 (a composite material composed of woven fiberglass cloth with an epoxy resin binder that is flame resistant), ceramic, glass, molding compound, or other supporting materials that may carry the conductive pads or lands needed to receive conductive terminals.

Substrate 130 may be made of a semiconductor wafer, or a portion of wafer. In some embodiments, substrate 130 includes silicon, gallium arsenide, silicon on insulator (“SOI”) or other similar materials. In some embodiments, substrate 130 also includes passive devices such as resistors, capacitors, inductors and the like, or active devices such as transistors. In some embodiments, substrate 130 includes additional integrated circuits. In addition, the substrate 130 may be made of other materials. For example, in some embodiments, substrate 130 is a multiple-layer circuit board. In some embodiments, substrate 130 also includes bismaleimide triazine (BT) resin, FR-4 (a composite material composed of woven fiberglass cloth with an epoxy resin binder that is flame resistant), ceramic, glass, plastic, tape, film, or other supporting materials that may carry the conductive pads or lands needed to receive conductive terminals.

Packaged die 110 is bonded to interconnect substrate 120 via connectors 115, and interconnect substrate 120 is bonded to substrate 130 via connectors 125. If two or more packaged dies, such as packaged die 110 and other packaged die(s), with different sizes of connectors are bonded to interconnect substrate 120, the packaging mechanisms could be challenging. Further, the cost of manufacturing the die package, such as die package 100, also needs to be taken into consideration. Interconnect substrates 120 with TSVs, which are also called interposers, provide functions for electrical connection and heat dissipation. However, interposers are expensive. For some applications that require low-cost die packages, alternative die package structures and methods for forming them are needed.

FIG. 2A shows a cross-sectional view of a die package 100′, in accordance with some embodiments. Die package 100′ includes a packaged die 110 _(A) and a packaged die 110 _(B). For example, packaged die 110 _(A) could be a central processing unit (CPU) or graphic control unit (GPU), and packaged die 110B could be a memory device, such as static random-access memory (SRAM) dynamic random-access memory (DRAM), or other types of memory devices. Packaged die 110 _(B) could have a large number of inputs/outputs (I/Os). As a result, the external connectors for packaged die 110 _(B) are small bumps, such as micro-bumps (μ-bumps). Packaged wdie 110 _(A) has connectors with different sizes. FIG. 2A shows packaged die 110 _(A) has large connectors and small connectors. The small connectors are about the same size as the connectors of packaged die 110 _(B). The large connectors of packaged die 110 _(A) are bonded directly to substrate (or package substrate) 130′ to form bonding structures 115 _(A). The small connectors of packaged die 110 _(A) and packaged die 110 _(B) are bonded to an interconnect substrate 120′ to form bonding structures 115 _(B). The interconnect substrate 120′ is electrically connected to interconnect structure 135 of substrate 130′ via connectors 125′. FIG. 2A also shows external connectors 138′ bonded to substrate 130′.

FIG. 2B shows a top view of die package 100′ of FIG. 2A, in accordance with some embodiments. FIG. 2B shows that packaged die 110 _(A) is placed next to packaged die 110 _(B). Interconnect substrate 120′ is disposed below packaged die 110 _(B) and a portion of packaged die 110 _(A). The bonding scheme shown in FIG. 2A does not involve a substrate with TSVs, whose manufacturing cost is high. As a result, the scheme in FIG. 2A saves manufacturing cost. Embodiments of mechanisms for forming die package 100′ are described below.

FIGS. 3A-3E illustrate cross-sectional views of a sequential flow of forming interconnect substrate 120′, in accordance with some embodiments. FIG. 3A shows a redistribution structure 302 formed over a substrate 301. As mentioned above, the substrate 301 for interconnect substrate 120′ may be made of a semiconductor wafer, glass, or other applicable materials. In some embodiments, substrate 301 includes silicon, glass, gallium arsenide, silicon on insulator (“SOT”) or other similar materials. FIGS. 3A-3E only show a region 300 of a single interconnect substrate 120′. During processing, a number of interconnect substrate 120′ are formed on substrate 301. At the end of the processing sequence, substrate 301 is sawed to separate into individual interconnect substrates 120′. Interconnect substrate 120′ does not contain active devices, such as transistors, memory devices, etc. However, interconnect substrate 120′ contains passive devices, such as resistors or capacitors, in some embodiments. As a result, the manufacturing cost of interconnect substrate 120′ is relative low and lower than an interposer with TSVs.

FIGS. 3A-3E show region 300 of interconnect substrate 120′, in accordance with some embodiments. During processing, the substrate 301 includes a number of regions similar to region 300 for forming additional interconnect substrates 120′. The redistribution structure 302 includes one or more redistribution layers (RDLs), which are insulated by passivation layers. Examples of redistribution structures and bonding structures, and methods of forming them are described in U.S. application Ser. No. 13/427,753, entitled “Bump Structures for Multi-Chip Packaging,” filed on Mar. 22, 2012 (Attorney Docket No. TSMC2011-1339), and U.S. application Ser. No. 13/338,820, entitled “Packaged Semiconductor Device and Method of Packaging the Semiconductor Device,” filed on Dec. 28, 2011 (Attorney Docket No. TSMC2011-1368). Both above-mentioned applications are incorporated herein by reference in their entireties.

FIG. 3A also shows that corners 303 are formed by removing portions of substrate 301. In some embodiments, corners 303 are removed by laser (a laser-grooving process), which removes trenches in substrate 301. Other material-removal process may also be used. Region 300 includes corners 303, which are half of the trenches. FIG. 3A shows that each of corners 303 has a slanted sidewall. In some embodiments, the angle, θ, between the slanted sidewall and a normal to the substrate surface is in a range from about 30 degrees to about 60 degrees. Corners 303 can be formed before or after the formation of redistribution structure 302. In some embodiments, corners 303 are formed after the formation of redistribution structure 302.

A plating seed layer 304 is then formed on redistribution structure 302 as shown in FIG. 3B in accordance with some embodiments. In some embodiments, the plating seed layer 304 is made of copper and is formed by physical vapor deposition (PVD). However, other conductive film may also be used. For example, the plating seed layer 304 may be made of Ti, Ti alloy, Cu, and/or Cu alloy. The Ti alloy and Cu alloy may include silver, chromium, nickel, tin, gold, tungsten, and combinations thereof. In some embodiments, the thickness of the plating seed layer 304 is in a range from about 0.1 μm to about 0.8 μm. In some embodiments, the plating seed layer 304 includes a diffusion barrier layer, which is formed prior to the deposition of the plating seed layer. The plating seed layer 304 may also act as an adhesion layer to an underlying layer. In some embodiments, the diffusion barrier layer is made of Ti with a thickness in a range from about 0.03 μm to about 0.1 μm. However, the diffusion barrier layer may be made of other materials, such as TaN, or other applicable materials and the thickness range is not limited to the range described above. The diffusion barrier layer is formed by PVD in some embodiments.

After plating seed layer 304 is formed, a photoresist layer 305 is defined over it, as shown in FIG. 3B in accordance with some embodiments. The photoresist layer 305 may be formed by a wet process, such as a spin-on process, or by a dry process, such as by applying a dry film over the surface of the plating seed layer 304. After the photoresist layer 305 is formed, the photoresist layer 305 is patterned to formed openings 306, which are used form connectors (or bonding structures, such as bumps) for the single interconnect substrate 120′. FIG. 3B also shows that photoresist layer 305 is also removed near corners 303 to form exposed regions 306′. The patterning processes involved include photolithography and resist development. In some embodiments, the width W₁ of openings 306 is in a range from about 10 μm to about 60 μm. In some embodiments, the depth D₁ of openings 306 is in a range from about 15 μm to about 80 μm.

Afterwards, a conductive layer 307 is plated on the surface of exposed plating seed layer 304, such as over the surfaces in openings 306 and over surfaces of exposed regions 306′, in accordance with some embodiments. The conductive layer 307 is made of copper, copper alloy, or a combination thereof in some embodiments. Following the formation of the first conductive layer 307, a solder layer 308 is formed over conductive layer 307. In some embodiments, both the conductive layer 307 and solder layer 308 are formed by plating processes. FIG. 3C shows the conductive layer 307 and solder layer 308 after their formation, in accordance with some embodiments. In some embodiments, the thickness of conductive layer 307 in openings 306 is in a range from about 10 μm to about 30 μm. In some embodiments, the thickness of solder layer 308 in openings 306 is in a range from about 5 μm to about 40 μm.

The thickness of conductive layer 307 and solder layer 308 over exposed regions 306′ are thicker than in openings 306 due to larger exposed surface area during plating processes. In some embodiments, the thickness of conductive layer 307 over exposed regions 306′ is in a range from about 12 μm to about 40 μm. In some embodiments, the thickness of solder layer 308 over exposed regions 306′ is in a range from about 5 μm to about 40 μm.

After the formation of the conductive layer 307 and solder layer 308, the photoresist layer 305 is removed, as shown in FIG. 3D in accordance with some embodiments. The removal process may involve dry or wet etching. An etch process is then performed to remove the plating seed layer 304 not covered by conductive layer 307 and solder layer 308.

After the photoresist layer 305 is removed and the etching of exposed plating seed layer 304, the conductive layer 307 and solder layer 308 in the openings 306 are exposed to form external connectors (or bump structures) 310. The exposed conductive layer 307 and solder layer 308 formed over the posed regions 306′ form contact structures 311.

A reflow process is then conducted to reflow the solder layer 308 over the patterned conductive layer 307 to prepare external connectors 310 for bonding. The solder layer 308 covering the conductive layer 307 near over exposed regions 306′ is also reflowed to cover side wall(s) of conductive layer 307, as shown in FIG. 3D in accordance with some embodiments.

Following the reflow process described above, substrate 301 is thinned down to a thickness T₁, as shown in FIG. 3E in some embodiments. The thinning process may apply a grinding process. In some embodiments, thickness T₁ is in a range from about 20 μm to about 50 μm μm. Following the thinning process, region 300 is singulated into individual piece from the entire substrate 301 for further packaging, to be described below. The singulation process is a sawing process, in accordance with some embodiments. In some embodiments, neighboring contact structures 311 of neighboring interconnect substrate 120′ are connected to one another prior to singulation process and are separated from one another after the singulation process. A portion of each contact structure 311 is in the scribe line, which is the region for sawing blade to cut through, for such embodiments.

FIGS. 4A-4E illustrate cross-sectional views of a sequential flow of forming packaged die 110 _(A), in accordance with some embodiments. FIG. 4A shows a redistribution structure 402 formed over a region 400 of substrate 401, which includes semiconductor devices (not shown), interconnect structures (not shown), and contact pads (not shown), which are formed over interconnect structures to make electrical connections with the semiconductor devices. The semiconductor devices may be active or passive. The interconnect structures may include metal layers and different layers of vias, which are used to connect metal layers. The conductive layers of the interconnect structures are insulated by dielectric layers. The redistribution structure 402 is formed over contact pads to make electrical connection to contact pads and semiconductor devices in substrate 401. FIGS. 4A-4E only show region 400 of a single die. During processing, a number of dies are formed on substrate 401. At the end of the processing sequence, substrate 401 is sawed to separate into individual packaged die 110 _(A).

The redistribution structure 402 includes one or more redistribution layers (RDLs), which are insulated by passivation layers. A plating seed layer 404 is then formed on redistribution structure 402 as shown in FIG. 4B in accordance with some embodiments. Plating seed layer 404 is similar to plating seed layer 304 described above. In some embodiments, the thickness of the plating seed layer 404 is in a range from about 0.1 μm to about 1.0 μm. In some embodiments, the plating seed layer 404 includes a diffusion barrier layer, which is formed prior to the deposition of the plating seed layer. The plating seed layer 304 may also act as an adhesion layer to under layer. In some embodiments, the diffusion barrier layer is made of Ti with a thickness in a range from about 0.01 μm to about 0.1 μm.

After plating seed layer 404 is formed, a photoresist layer 405 is deposited and patterned over it, as shown in FIG. 4B in accordance with some embodiments. The process of forming photoresist layer 405 is similar to the process of photoresist layer 305. The material used for forming photoresist layer 405 could also be similar to photoresist layer 305. Following the patterning of photoresist layer 405, a conductive layer 407 is plated on the surface of exposed plating seed layer 404, such as over the surfaces in openings (406). The conductive layer 407 is made of copper, copper alloy, or a combination thereof in some embodiments. In some embodiments, the thickness T₂ of conductive layer 407 formed in openings 406 is in a range from about 20 μm to about 80 μm. In some embodiments, the width W₂ of conductive layer 407 formed in openings 406 is in a range from about 60 μm to about 300 μm.

Afterward the conductive layer 407 is formed and the photoresist layer 405 is removed, as shown in FIG. 4C in accordance with some embodiments. The removal process may involve dry or wet etching. After the photoresist layer 405 is removed, the conductive layer 407 in the openings 406 is exposed. In some embodiments, the thickness T₂ of conductive layer 407 formed in openings 406 is in a range from about 20 μm to about 80 μm. In some embodiments, the width W₂ of conductive layer 407 formed in openings 406 is in a range from about 60 μm to about 300 μm.

After photoresist layer 405 is removed, a photoresist layer 408 is deposited and patterned over substrate 401, as shown in FIG. 4D in accordance with some embodiments. The process of forming photoresist layer 408 is similar to the process of photoresist layer 405. The material used for forming photoresist layer 408 could also be similar to photoresist layer 405. The patterns of the photoresist layer 408 include openings (409 _(I)) and (409 _(II)). FIG. 4D shows that the sizes of openings (409 _(I)) are substantially the same as the sizes of structures of conductive layer 407 of FIG. 4C. The sizes of openings (409 _(II)) are smaller than the sizes of openings (409 _(I)) to form smaller connectors (or bump structures). In some embodiments, the width W₃ of openings (409 _(II)) is in a range from about 50 μm to about 290 μm.

Following the patterning of photoresist layer 408, a conductive layer 410 and a solder layer 411 are plated on substrate 401 to fill at least portions openings (409 _(I)) and (409 _(II)), as shown in FIG. 4D. The conductive layer 410 is made of copper, copper alloy, or a combination thereof in some embodiments. The solder layer 411 is formed over the conductive layer 410. Due to difference in sizes of openings (409 _(I)) and (409 _(II)), the thicknesses of conductive layer 410 and solder layer 411 formed in these two types of openings are different. Plating process would grow thicker films in wider openings. In some embodiments, the thickness T₃ of conductive layer 410 formed in openings 409 _(I) is in a range from about 10 μm to about 60 μm, and the thickness T₄ of solder layer 411 in openings 409 _(I) is in a range from about 20 μm to about 40 μm. In some embodiments, the thickness T₅ of conductive layer 410 formed in openings 409 _(II) is in a range from about 12 μm to about 40 μm, and the thickness T₆ of solder layer 411 in openings 409 _(II) is in a range from about 5 μm to about 40 μm.

Afterward the solder layer 407 is deposited and the photoresist layer 408 is removed, as shown in FIG. 4E in accordance with some embodiments. The removal process may involve dry or wet etching. After the photoresist layer 408 is removed, an etch process is performed to remove plating seed layer 404 not covered by the conductive layers 407, 410, and solder layer 411. The conductive layer 407, conductive layer 410, and solder layer 411 in the openings (409 _(I)) are exposed to form external connectors (or bump structures) 412. Similarly, the conductive layer 410 and solder layer 411 in openings (409 _(II)) are also exposed to form connectors (or bump structures) 413. A reflow process is then performed to prepare the external connectors 412 and 413 for bonding. FIG. 4E shows the connectors 412 and 413 after the reflow process, in accordance with some embodiments.

Following the reflow process described above, region 400 is singulated into individual piece from the entire substrate 401 and becomes packaged die 110 _(A), which is ready for further packaging. The sigulation process is a sawing process, in accordance with some embodiments.

Packaged die 110 _(B) have one-size external connectors, as shown in FIG. 2A. The process sequence for forming external connectors of packaged die 110 _(B) can be extracted from the process flows described in FIGS. 3A-3E and FIGS. 4A-4E.

After interconnect substrates 120′, packaged dies 110 _(A), and packaged dies 110 _(B) are prepared or provided, they are assembled on substrates 130′. FIGS. 5A-5D illustrate cross-sectional views of a sequential flow of forming die package 100′, in accordance with some embodiments. FIG. 5A shows that a substrate 130′ is provided. Substrate 130′ includes a number of bonding structures 501, which are formed over interconnect structure 505. In some embodiments, bonding structures 501 are bonding pads. In some embodiments, there is a solder layer over the bonding pads on each of the bonding structures 501, which become bump structures after subsequent processing. Interconnect structure 505 includes conductive layers, such as metal layers 503, and vias 504, such as plating through holes (PTHs), formed in dielectric material(s) in accordance with some embodiments. Vias 504 are electrically connected to bonding pads 506 on the other side substrate 130′. Connectors would be formed on bonding pads 506 at a later stage, which will be described below. In some embodiments, substrate 130′ includes dielectric material(s) made of a composite material composed of woven fiberglass cloth with an epoxy resin binder that is flame resistant.

Substrate 130′ also includes an opening 502 to house interconnect substrate 120′. FIG. 5B shows the interconnect substrate 120′ being placed in opening 502 and being connected to the interconnect structure 505 of substrate 130′. FIG. 5B shows that solder balls 125′ are placed in a space between the interconnect structure 505 and substrate 130′. Solder balls 125′ are soldered to neighboring conductive structure of interconnect structure 505 and contact structures 311 of interconnect substrate 120′ to physically and electrically connect interconnect substrate 120′ with substrate 130′, in accordance with some embodiments.

After the interconnect substrate 120′ is bonded to substrate 130′, packaged dies 110 _(A) and 110 _(B) are bonded to interconnect substrate 120′ and substrate 130′, as shown in FIG. 5C in accordance with some embodiments. Either packaged die 110 _(A) or packaged die 110 _(B) can by bonded first. In addition, after one packaged die is bonded, an electrical test can be conducted to ensure the bonding of the packaged die is good before the other packaged die is bonded. For example, packaged die 110 _(A) is picked and placed over substrate 130′ to be bonded to bonding structures 501 to form bonded structures 115 _(A) and a portion of external connectors (or bump structures) 310 of interconnect substrate 120′ to form bonded structures 115 _(B). The bonding process involves solder reflow. Afterwards, an electrical test is conducted to ensure the bonding of packaged die 110 _(A) yields good results before packaged die 110 _(B) is bonded to the remaining connectors 310 of interconnect substrate 120′, in some embodiments. The electrical test enables screening of poorly bonded packaged dies to prevent wasting additional resources, such as packaged dies 110 _(B), by bonding to known bad packaged structures.

After the electrical test is done, packaged die 110 _(B) is bonded to the remaining connectors 310 of interconnect substrate 120′ to form bonded structures 115 _(B), in some embodiments. However, the electrical test can be optional. In some embodiments, another electrical test is performed after packaged die 110 _(B) is bonded. This other electrical test can check the quality of bonding of packaged die 110 _(B) to reduce waste of resources. After both packaged dies 110 _(A) and 110 _(B) are bonded to substrate 130′ and interconnect substrate 120′, a molding compound 512 is applied over substrate 130′ to cover packed dies 110 _(A) and 110 _(B) and to fill the space underneath packaged dies 110 _(A) and 110 _(B), a shown in FIG. 5D in accordance with some embodiments. In some embodiments, an underfill (not shown) is applied to fill the space under packaged dies 110A and 110B before molding compound 512 is applied. A thermal reflow process is performed to set the molding compound 512. If an underfill is applied, a thermal reflow process is also performed immediately afterwards to set the underfill.

After the molding compound 512 is formed, external connectors (such as solder balls) 138′ are formed on bonding pads 506 to form die package 100′, as shown in FIG. 5D in accordance with some embodiments. The process could involve turning substrate 130′ upside down and placing substrate 130′ on a glue layer (not shown) with molding compound 512 contacting the glue layer. After substrate 130′ is secured to the glue layer, solder balls 138′ are placed over bonding pads 506 and are bonded to bonding pads 506 by reflow. Die package 100′ is then singulated to be separated from other did packages 100′ of substrate 130′. FIG. 5D shows die package 100′ in accordance with some embodiments.

FIG. 6 shows a cross-sectional view of a die package 100″, in accordance with some embodiments. Die package 100″ includes a packaged die 110 _(C) and a packaged die 110 _(D). Both packaged die 110 _(C) and packaged die 110 _(d) have large numbers of inputs/outputs (I/Os). As a result, the external connectors for them are small bumps, such as micro-bumps (μ-bumps). Both packaged dies 110 _(C) and 110 _(D) are bonded to an interconnect substrate 120″ to form bonding structures 115″. A glue layer 610 is used to adhere interconnect substrate 120″ to substrate (or package substrate) 130″. The interconnect substrate 120″ is electrically connected to interconnect structure 135′ of substrate 130′ via connecting devices, such as wire bonds 125″. Other types of connecting devices, such as solder balls 125′ described above may also be used. An opening similar to opening 502 described above to house interconnect substrate 120′ may also be formed to accommodate interconnect substrate 120″. FIG. 6 also shows external connectors 138″ bonded to substrate 130″.

The formation mechanisms for interconnect substrate 120″ are similar to those of interconnect substrate 120′. The formation mechanisms for packaged dies 110 _(C) and 110 _(D) are similar to the formation mechanisms of packaged die 110 _(B) described above. Substrate 130″ is similar to substrate 130′; however, the interconnect structures and bonding structures on substrate 130″ could be arranged differently from substrate 130′.

After interconnect substrate 120″, packaged die 110 _(C), and packaged die 110 _(D) are prepared or provided, they are assembled on substrate 130″. FIGS. 7A-7E illustrate cross-sectional views of a sequential flow of forming die package 100″, in accordance with some embodiments. FIG. 7A shows a packaged die 110 _(C) is picked and placed over a interconnect substrate 120″ to be bonded to substrate 120″. Packaged die 110 _(C) is then bonded to interconnect substrate 120″. Electrical testing (or probing) is then conducted to test the quality of bonding and to test the quality of packaged die 110 _(C) by electrical probes 710, as shown in FIG. 7B in accordance with some embodiments. However, the electrical testing is optional.

Afterwards, interconnect substrate 120″ is attached to substrate 130″, such as by a glue layer (not shown), as mentioned above. In addition, electrical connection is made between interconnect substrate 120″ and substrate 130″. FIG. 7C shows that the electrical connection is made by wire bonds 125″, in accordance with some embodiments. Following making the electrical connection, a packaged die 110 _(D) is placed over interconnect substrate 120″ to be bonded to it, as shown in FIG. 7D in accordance with some embodiments.

After packaged die 110 _(D) is bonded to interconnect substrate 120″, a molding compound 712 is formed over substrate 130″ to protect packaged dies (110 _(C) and 110 _(D)) and substrate (120″) and connecting structures (bonding structures between packaged dies and substrate 120″, and wire bonds 125″) over substrate 130″. In some embodiments, an underfill is first formed under packaged dies 110 _(C) and 110 _(D) prior to forming molding compound 712. However, forming the underfill first is optional. Some molding compound materials can also act as underfill to fill the space between packaged dies 110 _(C)/110 _(D) and substrate 120″. After the molding compound 712 is formed, external connectors 138″ are formed on the opposite side (opposite from bonded packaged dies 110 _(C) and 110 _(D)) to form die package 100″, as shown in FIG. 7E. As mentioned above, each substrate 130″ could include a number of die packages. Die packages 100″ are then singulated into individual pieces. FIG. 7E shows die package 100″ after it has been singulated.

The process flow described above to form die package 100″ is merely one embodiment. Other process flow may also be used. For example, interconnect substrate 120″ could have been placed on substrate 130″ first before packaged dies 110 _(C) and 110 _(D) being bonded to substrate 120″. Further, packaged die 110 _(D) could have been bonded to interconnect substrate 120″ before packaged die 110 _(C). Choosing which die to bond first depends on the components on die package 100″ and how these components are used. For example, packaged die 110 _(C) may be bonded first because the testing of packaged die 110 _(D) could require the presence of package die 110 _(C). Other considerations may be needed in deciding the sequence of bonding and whether to conduct electrical testing in the sequence of forming die package 100″.

The embodiments described above show two packaged dies bonded in each die package, such as packaged dies 110 _(A) and 110 _(B) on die package 100′ or packaged dies 110 _(C) and 110 _(D) on die package 100″. There could be more than two packaged dies on each die package. FIG. 8 shows a top view of a die package 100* with three packaged dies, 110 _(B), 110 _(F), and 110 _(G), bonded an interconnect substrate 120*, which is bonded to a substrate 130*, in accordance with some embodiments. Interconnect substrate 120* is similar to interconnect substrate 120″ described above and substrate 130* is similar to substrate 130″ described above. The cross-sectional view of die package 100* is similar to the cross-sectional view of die package 100″ of FIG. 6. Higher number of die packages, such as 4, 5, or more, could be arranged and connected to the interconnect substrate 120* similar to substrate 120′ or 120″ described above and be directly or indirectly connected to a substrate 130* similar to substrate 130′ or 130″ described above.

As mentioned above in the description for FIGS. 5C, 7B and 7D, after one or more packaged dies, such as 110 _(A), 110 _(B), 110 _(C), and/or 110 _(D), are bonded to interconnect substrate 120′ or 120″, electrical tests can be conducted to test the quality of the bonding and also possibly the functionalities of the bonded packaged die(s). In some embodiments, interconnect substrate 120′ has a number of probing pads 910 that are not covered by packaged dies 110 _(A) and 110 _(B), as shown in FIG. 9A in accordance with some embodiments. FIG. 9A shows a top view of FIG. 5C, in accordance with some embodiments. In some embodiments, probing pads 910 are formed by the top metal layer of RDL of redistribution structure 302 (see FIG. 3E). In some other embodiments, areas of probing pads 910 are opened along with openings 306 and probing pads 910 are made of conductive layer 307 or a combination of conductive layer 307 and solder layer 308. Interconnect structures are formed between probing pads 910 and packaged dies 110 _(A) and/or 110 _(B) to enable electrical testing. FIG. 9A shows that the probing pads 910 are placed on a surface of interconnect substrate 120′ not covered by the packaged dies 110 _(A) and 110 _(B), and are close to packaged die 110 _(B) (small die). However, probing pads 910 can be placed in different places. For example, some probing pads 910 can be placed to surround packaged die 110 _(A) to allow shorter connections to devices in packaged die 110 _(A), as shown in FIG. 9B in accordance with some embodiments.

Probing pads 910 can be used to test the quality and connectivity of the bonding structures formed between packaged dies 110 _(A), 110 _(B), and interconnect substrate 120′. To test connectivity between signal structures and power-source structures or between signal structures and ground structures, metal lines are needed to connect these structures. However, the electrical connections used for testing need to be disconnected after testing is completed to allow the devices in the packaged dies 110 _(A) and 110 _(B) to work. FIG. 10A shows electrical connections between probing pads 910 _(A), 910 _(B), 910 _(C), 910 _(D) and bonding structures 115 _(B) of packaged dies 110 _(B) and/or 110 _(A), in accordance with some embodiments. Probing pads 910 _(A) and 910 _(C) connect bonding structures 115 _(B) that are connected to “Signal 1”, “Ground 1”, “Ground 2”, and “Signal 3” structures via metal lines 920. Metal lines 920 are RDLs of redistribution structure 302 described above. The metal lines 920 contact bonding structures 115 through external connectors (or bump structures) 310 of interconnect substrate 120″.

By placing electrical-test probers with probing pads 910 _(A) and 910 _(C) and supplying current and/or voltage between probing pads 910 _(A) and 910 _(C) various electrical tests can be performed. For example, to test the quality of bonding, a current can be applied between probing pads 910 _(A) and 910 _(C). The voltages of probing pads 910 _(A) and 910 _(C) are then measured to calculate the resistance between probing pads 910 _(A) and 910 _(C). The value of the resistance measured would reflect the quality of bonding. A value higher than an expected range could indicate improper bonding, such as cracking, mis-alignment, etc. The structures connected between probing pads 910 _(A) and 910 _(C) enable testing the connection to the grounding and signal structures.

Similarly, probing pads 910 _(B) and 910 _(C) connect bonding structures 115 _(B) that are connected to “Signal 2”, “Power 1”, “Power 2”, and “Signal 4” structures through electrical lines 925 (lines with circles). By placing electrical-test probers with probing pads 910 _(B) and 910 _(D) and supplying current and/or voltage between probing pads 910 _(B) and 910 _(D) various electrical tests can be performed. For example, to test the quality of bonding, a current can be applied between probing pads 910 _(B) and 910 _(D). The voltages of probing pads 910 _(B) and 910 _(D) are then measured to calculate the resistance between probing pads 910 ₁₃ and 910 _(D). The value of the resistance measured would reflect the quality of bonding. A value higher than an expected range could indicate improper bonding, such as cracking, mis-alignment, etc. The structures connected between probing pads 910 ₁₃ and 910 _(D) enable testing the connection to the power and signal structures.

FIG. 10A shows that metal line 920 _(A) of metal lines 920 cross metal lines 925 _(A), 925 _(B), 925 _(C), 925 _(D) of metal lines 925 from a top view. To avoid these lines crossing each other, they can be placed at different RDL level. For example, metal line 920 _(A) could be placed at an RDL level, which is below or above the RDL for metal lines 925 _(A), 925 ₁₃, 925 _(C), 925 _(D). Similarly, metal line 925 _(A) cross a few metal lines for connecting probing pads 910A and 910C from a top view. Metal line 925 _(A) may be place at different RDL level from the metal lines it might cross if they are placed on the same RDL level.

Metal lines 920 could be on the same RDL level or different RDL levels. Similarly, metal lines 920 could be on the same RDL level or different RDL levels. As described above, multiple levels of RDLs could be used to avoid metal line crossing.

The structures in FIG. 10A described are called daisy-chain structures. They are useful in testing quality of bonding between packaged dies and substrate. The structures described are merely examples, there could be other types of daisy-chain structures. After the testing is completed, the metal lines, such as metal lines 920 and 925, between metal pads 910 _(A), 910 _(B), 910 _(C), 910 _(D), and bonding structures 115 _(B) need to be severed (or cut) to allowed devices in packaged dies 110 _(A) and 110 _(B) to function properly. FIG. 10B shows severed metal lines 920, 925 between metal pads 910 _(A), 910 _(B), 910 _(C), 910, and bonding structures 115 _(B), in accordance with some embodiments. In some embodiments, metal lines 920, 925 are severed by laser. Portions of metal lines 920, 925 are exposed to enable severing by a severing tool, such as laser. Laser melts metal lines 920, 925 and separates them into discontinued pieces.

FIGS. 11A-11C show cross-sectional views of a sequential flow of testing and bonding packaged dies to form die package 100′, in accordance with some embodiments. FIGS. 11A-11C illustrate additional details of process and structures described above of FIG. 5C, in accordance with some embodiments. FIG. 11A shows that after packaged die 110 _(A) is bonded to interconnect substrate 120′ and substrate 130′, an electrical test can be conducted to ensure the bonding of the packaged die 110 _(A) is good before the other packaged die is bonded. FIG. 11A shows that probers 1110 are lowered to contact probe pads 910 described above. Probing pads 910 are on the top metal layer to allow contact with probers 1110. However, this operation is optional.

Afterwards, packaged die 110 _(B) is bonded to interconnect substrate 120′. Probers 1120 are then lowered to contact probe pads 910 to make electrical tests, as shown in FIG. 11B in accordance with some embodiments. The probe pads 910 contacted by probers 1120 could be the same or different from the probe pads contacted by probers 1110. Some of the electrical tests involve daisy-chain structures with probe pads 910 connected to bonding structures 115B and metal lines 920, 925 described above. After the test is completed, the interconnecting metal lines, such as 920, 925, are severed (or cut, or separated), as shown in FIG. 11C. In some embodiments, the interconnecting metal lines (such as 920, 925) are cut by laser, which is produced by a severing tool 1130. Following the severance, molding compound 512 and external connectors 138′ are formed, as described in FIG. 5D in accordance with some embodiments.

Traditionally, the quality of bonding of packaged dies can be tested after the die package is completed. The test structures and the probing structures on interconnect substrate described above enable testing without waiting until the die package is completed assembled. If electrical data reveal that packaged dies or bonding structures are less than perfect, they can be removed and be replaced by new packaged dies with newly formed bonding structures. However, such rework is not possible for finished die package, when the molding compound is formed. The mechanisms for forming test structures with probing pads and severing connecting metal lines after testing could reduce manufacturing cost.

Embodiments of mechanisms for testing a die package with multiple packaged dies on a package substrate use an interconnect substrate to provide electrical connections between dies and the package substrate and to provide probing structures (or pads). Testing structures, including daisy-chain structures, with metal lines to connect bonding structures connected to signals, power source, and/or grounding structures are connected to probing structures on the interconnect substrate. The testing structures enable determining the quality of bonding and/or functionalities of packaged dies bonded. After electrical testing is completed, the metal lines connecting the probing structures and the bonding structures are severed to allow proper function of devices in the die package. The mechanisms for forming test structures with probing pads on interconnect substrate and severing connecting metal lines after testing could reduce manufacturing cost.

In some embodiments, a semiconductor die package is provided. The semiconductor die package includes a first packaged die, and an interconnect substrate with a redistribution structure. The first packaged die is bonded to the redistribution structure, and the interconnect substrate includes a plurality of probing pads whose electrical connections to the first packaged die being severed. The semiconductor die package also includes a package substrate with an interconnect structure. The interconnect substrate is bonded to the package substrate, and wherein the package substrate is electrically connected to the first packaged die.

In some other embodiments, a method of forming a semiconductor die package is provided. The method includes bonding an interconnect substrate to a package substrate, and bonding a first packaged die to the package substrate and to the interconnect substrate. The method also includes bonding a second packaged die to the interconnect substrate, and performing an electrical test on a plurality of test structures. The plurality of test structures include a plurality of probing pads electrically connected to devices in the first packaged die and the second packaged die by metal lines. The method further includes severing the metal lines to terminate electrical connection between the plurality of probing pads, the first packaged die, and the second packaged die.

In yet some other embodiments, a method of forming a semiconductor die package is provided. The method includes bonding an interconnect substrate to a package substrate, and bonding a first packaged die to the package substrate and to the interconnect substrate. The method also includes bonding a second packaged die to the interconnect substrate, and performing an electrical test on a plurality of test structures. The plurality of test structures include a plurality of probing pads electrically connected to devices in the first packaged die and the second packaged die by metal lines. The method further includes severing the metal lines to terminate electrical connection between the plurality of probing pads, the first packaged die, and the second packaged die. In addition, the method includes forming a molding compound over the packaged substrate to cover the first packaged die and the second packaged die bonded to the package substrate and the interconnect substrate. Additionally, the method includes forming external connectors of the package substrate.

In an embodiment, a semiconductor die package is provided. The package includes a first packaged die bonded to an interconnect substrate by a first plurality of bonding structures and a second packaged die bonded to the interconnect substrate by a second plurality of bonding structures. The package further includes a plurality of test structures, wherein the plurality of test structures comprises a plurality of probing pads, a first test structure of the plurality of test structures being electrically isolated from the first plurality of bonding structures, the second plurality of bonding structures, and other ones of the plurality of probing pads.

In yet another embodiment a semiconductor die package is provided. The package includes a first packaged die bonded to an interconnect substrate by a first plurality of bonding structures and a second packaged die bonded to the interconnect substrate by a second plurality of bonding structures. The package further includes a test structure, wherein the test structure comprises a probing pad, a first line section and a second line section, the first line section having a first end and a second end, the first end terminating at the probing pad, the second line section having a third end and a fourth end, the third end terminating at one of the first plurality of bonding structures or one of the second plurality of bonding structures, the fourth end and the second end being disconnected and aligned. A molding compound is formed over the first packaged die, the second packaged die, and the interconnect substrate.

In yet still another embodiment a semiconductor die package is provided. The package includes a first substrate bonded to an interconnect substrate by a first plurality of bonding structures and a second substrate bonded to the interconnect substrate by a second plurality of bonding structures. The package further includes a plurality of test structures on the interconnect substrate, wherein each of the plurality of test structures comprise a probing pad connected to a discontinuous conductive line, a first segment of the discontinuous conductive line coupled to a corresponding one of the first plurality of bonding structures or a corresponding one of the second plurality of bonding structures, a second segment of the discontinuous conductive line coupled to the probing pad, terminating ends of the first segment and the second segment being separated by a gap. A molding compound is formed over the first substrate, the second substrate, and the interconnect substrate.

Although embodiments of the present disclosure and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. For example, it will be readily understood by those skilled in the art that many of the features, functions, processes, and materials described herein may be varied while remaining within the scope of the present disclosure. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

What is claimed is:
 1. A method of forming a semiconductor die package, the method comprising: bonding a first electrical connector on a sidewall of an interconnect substrate to a sidewall of a second electrical connector in a package substrate; bonding a first packaged die to the package substrate using a first bonding structure; bonding the first packaged die to the interconnect substrate using a second bonding structure; and bonding a second packaged die to the interconnect substrate using a third bonding structure.
 2. The method of claim 1, wherein bonding the first electrical connector to the second electrical connector comprises placing the interconnect substrate in a recess of the package substrate.
 3. The method of claim 1, wherein the first bonding structure and the second bonding structure are different sizes.
 4. The method of claim 1, further comprising forming the interconnect substrate, wherein forming the interconnect substrate comprises: forming a groove in a substrate; forming a redistribution structure over the substrate; forming a conductive layer over the redistribution structure, the conductive layer extending into the groove; thinning the substrate; and singulating the substrate along the groove to form the interconnect substrate, wherein the conductive layer within the groove forms the first electrical connector.
 5. The method of claim 4, wherein after singulating, at least a portion of the substrate is exposed below the conductive layer along a sidewall of the substrate.
 6. The method of claim 1, further comprising forming the first packaged die, forming the first packaged die comprising: forming a first mask layer over a die, the first mask layer having a first opening; forming a first conductive layer in the first opening; forming a second mask layer over the die, the second mask layer having a second opening and a third opening, the first conductive layer being exposed in the second opening; forming a second conductive layer in the second opening and the third opening; and removing the first mask layer and the second mask layer, wherein the first bonding structure is coupled to the second conductive layer formed in the second opening and the second bonding structure is coupled to the second conductive layer formed in the third opening.
 7. The method of claim 6, further comprising forming a solder material over the second conductive layer prior to removing the second mask layer.
 8. A method of forming a semiconductor die package, the method comprising: forming an interconnect substrate, wherein forming the interconnect substrate comprises: forming a redistribution layer over a first substrate, the first substrate having a first beveled edge; forming a seed layer over the redistribution layer and the first beveled edge of the first substrate; forming a mask layer over the seed layer, the mask layer having a plurality of openings; forming a first interconnect contact, a second interconnect contact, and a third interconnect contact in the plurality of openings, wherein the first interconnect contact and the second interconnect contact are on a major surface of the first substrate, wherein the third interconnect contact is on the first beveled edge of the first substrate; and removing the mask layer and exposed portions of the seed layer; bonding the third interconnect contact to a sidewall of a first package substrate contact of a package substrate; bonding a first packaged die contact of a first packaged die to a second package substrate contact of the package substrate; bonding a second packaged die contact of the first packaged die to the first interconnect contact of the interconnect substrate; and bonding a third packaged die contact of a second packaged die to the second interconnect contact of the interconnect substrate.
 9. The method of claim 8, wherein the first beveled edge extends lower than the redistribution layer.
 10. The method of claim 8, wherein a sidewall of the interconnect substrate comprises the first beveled edge and a lower portion extending from the first beveled edge to a bottom of the interconnect substrate, wherein the lower portion is free of the third interconnect contact.
 11. The method of claim 8, wherein bonding the third interconnect contact is performed at least in part using solder.
 12. The method of claim 8, further comprising encapsulating the first packaged die and the second packaged die with an encapsulant, the encapsulant being interposed between the interconnect substrate and the package substrate.
 13. The method of claim 8 further comprising forming a fourth interconnect contact on a second beveled edge of the first substrate.
 14. The method of claim 8, wherein the first packaged die contact has a first width, the second packaged die contact has a second width, the first width being different than the second width.
 15. A method of forming a semiconductor die package, the method comprising: attaching an interconnect substrate to a package substrate, wherein the interconnect substrate has a first electrical contact on a sidewall of the interconnect substrate, a second electrical contact on a major surface of the interconnect substrate, and a third electrical contact on the major surface of the interconnect substrate, the package substrate having a fourth electrical contact, wherein the first electrical contact of the interconnect substrate is bonded to a sidewall of the fourth electrical contact of the package substrate using a first electrical connector; bonding a first substrate to the second electrical contact of the interconnect substrate using a second electrical connector; bonding the first substrate to the package substrate using a third electrical connector; and bonding a second substrate to the third electrical contact of the interconnect substrate using a fourth electrical connector.
 16. The method of claim 15 further comprising forming a dielectric material between the first substrate and the interconnect substrate.
 17. The method of claim 16, wherein the dielectric material extends between the interconnect substrate and the package substrate.
 18. The method of claim 17, wherein the dielectric material extends along sidewalls of the first substrate and the second substrate.
 19. The method of claim 15, wherein the sidewall of the interconnect substrate comprises a beveled portion and a vertical portion, wherein the first electrical contact is on the beveled portion.
 20. The method of claim 15, wherein the second electrical contact is smaller than the third electrical contact. 